The present invention relates to coilable rules and more particularly to such rules employing coilable metal blades with greater standout length when unsupported as extended from the casing.
Coilable metal rules are widely used and frequently employ a concavo-convex cross section or other cross sectional configuration which will stiffen the blade so that the user may singlehandedly bridge a distance with the blade tip extended in a straight line. This enables the user to bridge doorways, shaftways, excavations and the like with the unsupported blade. It is known that the degree of curvature or cross section will influence and substantially determine the standout length (the length of the blade which can be extended unsupported without collapsing at the support point) or breakpoint (the point at which the unsupported rule will collapse) for a rule of any particular width, thickness and metal characteristics.
Such blades are generally used in power returnable coilable rules that include a coiled spring which has the effect of retracting the extended blade into the rule housing or casing wherein it coils about the reel in its retracted position. It will be appreciated that the concavo-convex cross section of the blade is flattened as it is coiled within the rule casing, whether by manual action or by the retracting effect of such a power return spring. The stiffer the blade, the more spring power required to effect such flattening of the blade upon retraction.
As is also well known, blades employing such a concavo-convex cross section generally must be heat treated to achieve the desired flexural characteristics to withstand the repeated flattening operation that occurs in the coiling of the blade within the rule casing. It has been necessary to balance the advantages to be gained by increasing the depth of curvature of the blade for increased length of blade standout with the power requirements for the spring to effect its retraction and also with the life characteristics resulting from repeated deflection into the flattened condition when the blade is coiled.
Moreover, because the extended blade will assume the concavo-convex cross section, the indicia which are imprinted thereon become more difficult to read when the arcuate cross section becomes steeper. Lastly, when there is a very high cross curve or deep cross section, there is a stress point at the point where the blade does buckle when fully extended, which, over repeated usage, may cause a stress fracture.
Because of the necessity for balancing these various factors, the range of average standout lengths for commercially available widths of concavo-convex blades is set forth in the following table:
______________________________________ BLADE WIDTH, INCHES STANDOUT LENGTH, INCHES ______________________________________ 1/4 12-16 1/2 35-43 3/4 55-73 1 86-97 ______________________________________
Various cross sections have been used for the concavo-convex blade construction. Generally, the cross section is a segment of a circular arc or a segment of a circular arc with straight segments at its ends (e.g, rectilinear portions which are tangential to the center arcuate segment). In this manner the concavo-convex cross section will not have any sharp bends which would produce points where the blade will be overstressed during the repeated flattening operation, and the amount of force required to flatten the curve is more readily available from economically utilizable power return springs.
It is an object of the present invention to provide a novel coilable metal blade for a rule which provides greater standout length.
It is also an object to provide such a blade which can be readily and economically fabricated and which will enjoy long life.
Another object is to provide such a blade which will slow the rate of retraction as the blade is almost fully retracted into the casing.
A further object is to provide a novel rule employing such a blade and which may be fabricated readily and relatively economically.
Yet another object is to provide a novel method for fabricating such coilable metal rule blades offering enhanced or great standout.